Studies On Biogenic Elements And The Distribution And Molecular Diversity Of Phytoplankton Resting Cells In Surface Sediments From The Central Bohai Sea

Surface sediments were collected from 30 stations of the central Bohai Sea in this study,and total organic carbon(TOC),total nitrogen(TN),total phosphorus(TP),bio-silicon(BSi),organic matter(OM)in surface sediment samples,and moisture content(MC)was measured.Sources of organic matter were discussed,and environmental pollution was assessed.The purpose of this study is to reveal the environmental quality of surface sediments in the central Bohai Sea.In addition,dinoflagellate cysts were observed under microscopy qualitatively and quantitatively.Eukaryotic algae in sediments were analyzed by the high-throughput sequencing.Geographical distribution of dinoflagellate cysts and eukaryotic algae in the central Bohai Sea was illustrated,and eutrophication statues and potential for harmful algal blooms were discussed.The results of this study provide a scientific basis for understanding the marine environmental quality of the Bohai Sea and the prevention of harmful algal blooms.The main findings of this study are as followings:(1)The contents of biogenic elements in the central sediments of the Bohai Sea were: TOC(0.21%~1.43%),TN(0.01%~0.12%),TP(0.01%~0.04%),BSi(0.08%~0.46%),OM(1.43%~9.42%),MC(18.24%~47.70%).TOC,TN,TP,OM and MC showed similar distribution characteristics,all of which were higher in the sea areas near the Bohai Bay.While BSi is higher in the middle of the Bohai Sea.(2)Organic matters in the surface sediments from the central Bohai Sea originated from mixture inputs of terrestrial and marine sources,dominating by terrestrial inputs based on the molar ratios of TOC/TN.Diatoms contributed a lot to the total primary productivity of the central and eastern Bohai Sea based on TOC/BSi ratios.According to the sediment quality evaluation,TOC and TN in sediments from coastal areas exceeded the Class II standard,which reflected that the surface sediments in the central Bohai Sea has been polluted to some extent.(3)Eighteen taxa of dinoflagellate cysts,including 12 species of autotrophic dinoflagellate cysts and 6 species of heterotrophic dinoflagellate cysts,were observed under microscopic observation.Cyst concentrations in each station ranged from 119.4 to 189.3 cysts/g D Wt with an average of 146 cysts/g D Wt.The average concentration of autotrophic dinoflagellate cysts was 44.9 cysts/g D Wt,and 101.1 cysts/g D Wt for the heterotrophic dinoflagellate cysts.The dominant cyst species were cysts of Protoperidinium,Gonyaulax and Scrippsiella trochoidea.(4)Cysts of some harmful/toxic dinoflagellates occurred widely in the central Bohai Sea,including Alexandrium spp.,Gonyaulax spinifera,Scrippsiella trochoidea,Gymnodinium spp.,Polykrikos spp.,Pheopolykrikos hartmannii,and Protoperidinium spp.Toxic and harmful dinoflagellate cysts were widely distributed in the sea areas from the mouth of the Bohai Bay to the mouth of the Luanhe River,which coupled with sea areas of high occurrences of algal blooms.The G:P ratios were lower in the eastern Bohai Sea,and low G:P ratios and high proportions of Protoperidinium cysts reflected the high nutrient levels and high primary productivity near the Bohai Strait.(5)Fifty-four species of the eukaryotic algae were obtained in surface sediments of the central Bohai Sea based on high-throughput sequencing,including 3 phyla,14 classes,and 45 genera.Dinophyta(dinoflagellate)was the most diversified phyla,including 1 classes,20 genera,and 25 species.Twenty species in 16 genera of 8 classes were analyzed in Ochrophyta.Nine species in 9 genera of 5 classes were identified in Chlorophyta.Dinophyta was the most dominant group in surface sediments from the central Bohai Sea.Many toxic and harmful eukaryotic algae were found in the northwestern,central and eastern waters of the Bohai Sea.High-throughput sequencing has some advantages on identification of some nano-sized resting cells or those unknown species,this method can get more kinds of species,and most of them can identify the classification level of species.However analysis of data from the high-throughput sequencing is based on the improvement of database.Some resting cells can not identified on the species even genus level.Therefore,it is advisable to use both methods of high-throughput sequencing and microscopic observation in species identification.